The invention relates to a process for degrease annealing in a furnace thin strip and foil made of aluminum and aluminum alloys.
In the roll forming of aluminum strip down to thin strip and foil--hereinafter denoted simply by the term foil-petrol-based lubricating oil or agueous lubricating oil emulsions are employed as aids to rolling, whereby today mainly paraffin based petrols containing additives such as palm seed oil, long chain alkyl alcohols, monocarboxylic acid alkyl-esters etc. are used. The foils which are coiled into rolls are then placed in large batch ovens or furnaces and annealed, i.e. heated at elevated temperatures, in dry air. This annealing operation produces a degreasing effect i.e. the surface of the foil is freed of the rolling lubricant lying on its surfaces. Another result of the annealing is that the foil is transformed from the hard rolled condition into a soft condition which is necessary for certain subsequent operations.
After a degrease anneal at temperatures higher than 250.degree. C.--both here and in the following specification the annealing temperature is to be understood as the metal temperature--it is found that the individual layers in the coil stick together to a greater or lesser degree. The sticking increases strongly with increasing annealing temperature. In addition, at temperatures above about 400.degree. C. the surface of the foil can become discolored. Rolls of foil which exhibit such sticking tendencies can be handled in further operations only with considerable difficulty.
Commercial degrease annealing treatments are, today, conducted in such a way that the rolls of foil are annealed in furnaces at a metal temperature of 250.degree.-300.degree. C., with annealing times of approximately 30 to 60 hours depending on the size of coil, being required to achieve complete removal of rolling lubricant residues. The occasional rejection of foil due to sticking has to be accepted with this practice. The relatively long annealing times lead periodically to a shortage of free capacity for annealing and, in addition, represent a more than negligible cost factor.